Such a process is described in Fluid Catalytic Cracking Technology and Operations, Joseph W. Wilson, 1997, PennWell Publishing Company, Tulsa, Okla. USA, pages 9–18 and 236–248. According to this publication the FCC product mixture is first separated in a main fractionator by means of distillation. The gas from the main fractionator overhead drum flows to a wet gas compressor. This is usually a two-stage machine. The first stage discharge is cooled and partially condensed in an interstage cooler and the resulting liquid and gas fractions are subsequently separated in an interstage separator drum. The liquid obtained in this separator drum is combined with the liquid obtained after the second stage compression and fed to a stripper. In this stripper, or de-ethanizer tower ethane and lighter materials are removed from the liquid feed. The gaseous fraction obtained in the stripper is supplied to an absorber. To this absorber also the compressed gaseous fraction obtained after the second compression is supplied. In the absorber the more heavy compounds are removed by contacting the gaseous fraction with an absorber fluid, also referred to as absorber oil or lean oil. The bottom product obtained in the stripper is fed to a debutanizer. As absorber fluid overhead liquid from the main fractionator or debutanizer bottoms liquid are used. Typically the temperature of the fluid overhead liquid is between 40 and 50° C. Absorber overhead gas flows to the secondary or sponge absorber. The sponge absorber is intended to recover gasoline range material (mostly C5s) still present in the gas leaving the primary absorber. The rich oil obtained in the sponge absorber is recycled to the main fractionator. Because of this recycle of lower boiling hydrocarbons present in the rich oil to the main fractionator an increase of gas to be handled by the compressor will result.
U.S. Pat. No. 5,034,565 describes a process as described above, wherein primary absorber and stripper are combined in one vessel. U.S. Pat. No. 4,431,529, U.S. Pat. No. 4,714,524 and U.S. Pat. No. 4,605,493 describe a process as described above illustrating embodiments wherein stripper and absorber are arranged as separate process steps/vessels. In the above processes debutanizer bottoms are used as absorber fluid.
A problem often encountered with the above described processes is that the capacity of the main fractionator, compressor, primary absorber and/or stripper are not high enough when the charge of FCC product mixture is increased. In other words, these unit operations may form a bottleneck when the capacity of the FCC unit increases. An increase of FCC product mixture can for example be the resultant of better FCC catalyst used or a steady increase in FCC reactor capacity.
The present invention provides a method to debottleneck the above described process or to provide such a process which requires smaller equipment.